Ramba"m (Maimonedes) Hilchot Kid. Hachodesh 2:8 says:
Afterwards - i.e., after [the witnesses'] testimony is substantiated - the head of the court declares, "It has been sanctified." And all the people respond, "It has been sanctified. It has been sanctified."
Why was it necessary for the people to respond twice "It has been sanctified". Would once not have sufficed? What's the meaning behind the double declaration?
The source of the Rambam is the Mishna, Rosh Hashana 2:7, and Rav Ovadia MiBartenura explains (based on the combination of Rav Pappa and Rav Nachman Bar Yitzchak's opinions in the Gemara Rosh Hashana 24a) that the declaration is structured according to the verses in Vayikra 23 2 and 44.
In verse 44, Moshe declares the holidays, and thus the start of the month that determines their day. That is represented by the head of the court making the declaration. In verse 2 the plural indicates that it is called twice, and a textual variance indicates that "you" (the congregation) do that declaration.
M'leches Sh'lomo to Sh'kalim 3:3 cites a R. Y'hosef as saying that that's simply how people speak:
כן הוא דרך הדבור שהשואל אומר הדבור פעם אחת והמשיב משיב לו שני פעמים וכדומה לזה מצינו גבי קדוש החדש במס׳ ר״ה ראש ב״ד אומר מקודש וכל העם עונין אחריו מקודש מקודש וכן דרך בני אדם לדבר שאחד אומר לחברו וכי דבר זה יפה בעיניך והוא משיב יפה יפה וכדומה לזה יש בדברי חכמים
Seemingly, according to him, the mishna on which your Rambam is based, in mentioning the repetition, is descriptive rather than prescriptive. That said, I don't know that Rambam himself would agree.
